1. Field
The subject invention is in the field of lifting surfaces particularly those incorporated in aircraft or aeronautical propulsion systems. More specifically it is in the field of wing tip devices used with aircraft wings to minimize induced drag and alleviate noise effects associated with concentrated vorticity wakes that trail from lifting surfaces.
2. Prior Art
The patents listed below constitute a representative listing of the prior art in this field.
______________________________________ U.S. Pat. No.: ______________________________________ 994,968 G. Barbaudy 1,050,222 A. M. McIntosh 1,841,921 Spiegel 2,576,981 Vogt 2,775,419 Hlobic 2,805,830 Zborowski 3,029,018 Floyd 3,128,371 Spaulding et al 3,270,988 C. D. Cone ______________________________________
The drag of an aircraft wing arises from a number of sources of which that associated with the trailing vortex system is a major portion-approaching one-half the total drag for a subsonic aircraft in optimum cruise flight. It has long been recognized that this so-called induced drag is directly associated with wing lift and load variation along the span. This condition corresponds to a flow which appears as vorticity shed downstream of the wing trailing edge (i.e., the trailing vortex system) that is particularly intense near the wing tip. The induced drag depends on the following parameters in a manner conveniently expressed as follows: EQU Induced Drag=K.sub.i (lift).sup.2 /.pi.q(span).sup.2
where
q is ##EQU1## and, .rho.=air density PA1 K.sub.i =induced drag factor
V=flight velocity
also,
The induced drag factor, K.sub.i depends on the spanwise loading and the configuration of the lifting system. For a planar wing the elliptic loading is optimum and K.sub.i =1.0. However, it is also known that the minimum induced drag is less for wing configurations with increased ratios of total trailing edge length to span. Examples include monoplanes with tip winglets or endplates, multiplanes of various types (e.g., biplane, triplane) and various forms of arched lifting surfaces, either open or closed. Also assorted tip devices involving the use of multiple surfaces have been proposed for application to monoplanes or multiplanes. Most of the above are not particularly efficient or useful for various reasons including excessive structural weight, high loads, concomitant drag sources, and operational limitations. Thus, with the exception of the monoplane with winglets, they find very little use today. Several forms of winglets are currently in use for special applications where span and operational space may be limited or where existing configurations may benefit from their application. However it has not been generally established that winglets are preferable to or more efficient than simple wing span extensions to limit induced drag. In many cases their benefit is marginal or even cosmetic.
The object of the present invention is to employ the basic principle whereby the ratio of trailing edge length to wing span is maximized, particularly near the wing tip where this strategy is most effective, thereby substantially reducing induced drag. A further objective is to maintain favorable structural and operational characteristics without creating other sources of drag. More generally, the aim is to provide a generic geometric concept which can be adapted to achieve various levels of drag reduction consistent with the design objectives and requirements for any application which uses wings or wing-like elements e.g., helicopters, propellers, etc., including non-aeronautical applications.